Thrombosis remains a significant barrier to the use of blood contacting medical devices for treatment of disease. Contact activation of blood plasma coagulation occurs via interactions between material surfaces and proteins of the contact activation complex. Recent results show that the prevailing mechanistic model of contact activation is inconsistent with measured adsorption properties of blood proteins and fails to account for relationships among activator surface properties, production of enzyme intermediates, and time to plasma coagulation in vitro. This work has identified key problems to be solved in order to propose a revised scheme for surface activation of plasma coagulation, with work proposed guided by this central hypothesis. Blood plasma coagulation is initiated by contact with material surfaces through a complex surface-catalyzed activation reaction that rapidly converts the zymogen FXII into a distribution of activated fragments exhibiting procoagulant and/or ordinary amidolytic activity. The distribution of fragments depends on activator surface chemistry and nanoscopic distribution of that chemistry. Procoagulant fragments stimulate proportional activation of subsequent steps of the intrinsic pathway, ultimately leading to proportional production of thrombin in the penultimate step of the coagulation cascade. This hypothesis differs from the conventional paradigm of material-induced blood coagulation in that it envisions contact activation as a non-specific, surface-induced event producing a variety of FXII activation products, some of which are capable of inducing coagulation. Furthermore, the hypothesis states that propagation of the cascade occurs through a series of self-limiting reactions. The hypothesis will be tested through four specific aims that utilize biochemical and surface analysis methods to quantify the amount and activity of FXII fragments formed by blood-surface contact. Results will be used to develop new materials with sub-micron spatially distributed chemistries and for which there is evidence improved hemocompatibility compared to materials having chemistries distributed at the macro scale. Prospectively designed biomaterials have been a long-sought objective of the biomaterials community and represent a new-generation of synthetic, hemocompatible materials for medical devices.